On Mon, Feb 22, 2021 at 8:46 AM Amit Langote <amitlangot...@gmail.com> wrote: > > On Fri, Feb 19, 2021 at 7:38 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Thu, Feb 18, 2021 at 11:05 AM Amit Langote <amitlangot...@gmail.com> > > wrote: > > > > > > > > It also occurred to me that we can avoid pointless adding of > > > > > partitions if the final plan won't use parallelism. For that, the > > > > > patch adds checking glob->parallelModeNeeded, which seems to do the > > > > > trick though I don't know if that's the correct way of doing that. > > > > > > > > > > > > > I'm not sure if's pointless adding partitions even in the case of NOT > > > > using parallelism, because we may be relying on the result of > > > > parallel-safety checks on partitions in both cases. > > > > For example, insert_parallel.sql currently includes a test (that you > > > > originally provided in a previous post) that checks a non-parallel > > > > plan is generated after a parallel-unsafe trigger is created on a > > > > partition involved in the INSERT. > > > > If I further add to that test by then dropping that trigger and then > > > > again using EXPLAIN to see what plan is generated, then I'd expect a > > > > parallel-plan to be generated, but with the setrefs-v3.patch it still > > > > generates a non-parallel plan. So I think the "&& > > > > glob->parallelModeNeeded" part of test needs to be removed. > > > > > > Ah, okay, I didn't retest my case after making that change. > > > > Greg has point here but I feel something on previous lines (having a > > test of glob->parallelModeNeeded) is better. We only want to > > invalidate the plan if the prepared plan is unsafe to execute next > > time. > > > > It is quite possible that there are unsafe triggers on different > > partitions and only one of them is dropped, so next time planning will > > again yield to the same non-parallel plan. If we agree with that I > > think it is better to add this dependency in set_plan_refs (along with > > Gather node handling). > > Are you saying that partitions shouldn't be added to the dependency > list if a parallel plan was not chosen for insert into a partitioned > table for whatever reason (parallel unsafe expressions or beaten by > other paths in terms of cost)? >
Right. > If so, I am inclined to agree with > that. > > I may be wrong but it doesn't seem to me that the possibility of > constructing a better plan due to a given change is enough reason for > plancache.c to invalidate plans that depend on that change. AIUI, > plancache.c only considers a change interesting if it would *break* a > Query or a plan. > that makes sense to me. > So in this case, a non-parallel plan may be slower, but it isn't > exactly rendered *wrong* by changes that make a parallel plan > possible. > Right. > > Also, if we agree that we don't have any cheap way to determine > > parallel-safety of partitioned relations then shall we consider the > > patch being discussed [1] to be combined here? > > Yes, I think it does make sense to consider the GUC patch with the > patches on this thread. > Cool, Greg/Hou, can we consider this along with the next patch? -- With Regards, Amit Kapila.
